The phenomenon of muscle migration is of potential significance and use in orthopedic medicine, developmental biology, and evolutionary morphology. The work proposed in this application is designed to get at the control factors determining the age changes in migration of soft structures attached to long bones during growth. Work completed has shown that soft structures attached to bones retain constant positional relationships during growth. The best explanatory model at the present time is that these structures are hitchhikers on an elastic expanding periosteum that increases in length as the bone grows. Increase is equal at all positions. Attachment sites in young animals are quite different from those in older animals in that in young animals, the structures are attached only to the periosteum and not to the bone itself except secondarily through the medium of the perisoteum. Paralysis of muscles, alterations in their direction of pull, do not affect migration. Alterations in growth of the ends of the bone affect the attached structures in ways predicted by this model. Work during the coming year should allow us to determine the factors controlling the position of initial attachment, should improve our understanding of age changes in the attachments and the periosteum, and should improve out understanding of the process of attachment and migration.